Conventionally, noble metallic powders such as those of silver, palladium, platinum, and gold or base metallic powders such as those of nickel, cobalt, iron, molybdenum, and tungsten are used in conductive pastes as electrical materials, and in particular, in internal electrodes of multilayered ceramic capacitors. Generally, a multilayer ceramic capacitor is made by alternately laminating ceramic dielectric layers and metallic layers which are used as internal electrodes, and by forming external electrodes which are connected to the metallic layer of internal electrodes on two outside faces of the ceramic dielectric layers. Materials which contains a high dielectric constant material as a main component such as barium titanate, strontium titanate, and yttrium oxide are used in the dielectric substances. On the other hand, the noble metallic powders or the base metallic powders mentioned above may be used to form the internal electrodes. However, since more inexpensive electrical materials are required recently, multilayer ceramic capacitors in which the latter base metallic powders are used have been greatly developed. In particular, nickel powder has been mainly developed.
A multilayer ceramic capacitor may be made by the following method. First, a dielectric powder such as one of barium titanate is mixed with an organic binder so as to be suspended. Next, the suspension is formed into a sheet by a doctor blade method to form a dielectric green sheet. On the other hand, metallic powder for an internal electrode is mixed with organic compounds such as organic solvent, plasticizer, or organic binder to form a metallic powder paste, and this paste is coated on the green sheet mentioned above by screen printing. After they are dried, laminated, and pressed, the organic component is removed by heat treatment. Furthermore, they are burned at 1000 to 1300° C. or at more than 1300° C., and outer electrodes are baked on both sides of the dielectric ceramic layer to obtain a multilayer ceramic capacitor.
In the process for production of the multilayer ceramic capacitor described above, the heating treatment in which organic components are vaporized and removed after the laminating process and the pressing process is ordinary performed at 250 to 400° C. in air. In this way, since the heating treatment is performed under an oxidizing atmosphere, metallic nickel powder is oxidized, and therefore, volume expansion occurs. At the same time, the metallic nickel powder begins to be sintered, and volume shrinkage occurs.
Due to this, in the process for production of the multilayer ceramic capacitor, volume change occurs due to expansion or shrinkage of the metallic powder by oxidizing, reducing, and sintering reaction beginning at a low temperature of about 300° C. In this case, if the oxidation behavior or sintering behavior of the metallic nickel powder at low temperature is unstable, deformation may easily occur between the dielectric layer and the electrode layer. As a result, breaking of the layer structure, which is called delamination, such as cracking or exfoliation, may occur.
Many techniques have been suggested to overcome the delamination problem. For example, Japanese Unexamined Patent Application Publication No. Hei 08-246001 discloses metallic nickel power having a tap density of more than a certain limit value versus a certain particle diameter. In this publication, by producing a capacitor in which such a metallic nickel powder and a dielectric is contained in a paste thereof, delamination is unlikely to occur.
The above-mentioned conventional technique is efficient for improving sintering behavior to some extent; however, it is not sufficient to prevent delamination, and further improvement is required.